A quadrupole can be used as a mass filter such that ions of only a certain range of mass-to-charge ratios (also referred to as mass) are transmitted through the quadrupole. Such ions are considered to have a stable trajectory. Ions having a mass-to-charge ratio that is outside the stability range are filtered out. The stability range can be varied over time in a scan, thereby providing a mass spectrum over the scanned mass range.
Stability limits are set via applied AC and DC potentials that are capable of being ramped as a function of time such that ions with a specific range of mass-to-charge ratios have stable trajectories throughout the device. In particular, by applying fixed and/or ramped AC and DC voltages to configured cylindrical or hyperbolic electrode rod pairs, desired electrical fields stabilize the motion of predetermined ions in the x and y dimensions. As a result, the applied electrical field in the x-axis stabilizes the trajectory of heavier ions, whereas the lighter ions have unstable trajectories. By contrast, the electrical field in the y-axis stabilizes the trajectories of lighter ions, whereas the heavier ions have unstable trajectories. The range of masses that have stable trajectories in the quadrupole and thus arrive at a detector placed at the exit cross section of the quadrupole rod set is defined by the mass stability limits. In a typical operation, by varying the mass stability limits monotonically in time, the mass-to-charge ratio of an ion can be (approximately) determined from its arrival time at the detector.
In a conventional quadrupole mass spectrometer, the uncertainty in estimating of the mass-to-charge ratio from its arrival time corresponds to the width between the mass stability limits. This uncertainty can be reduced by narrowing the mass stability limits, i.e. operating the quadrupole as a narrow-band filter. In this mode, the mass resolving power of the quadrupole is enhanced as ions outside the narrow band of “stable” masses crash into the rods rather than passing through to the detector. However, the improved mass resolving power comes at the expense of sensitivity. In particular, when the stability limits are narrow, even “stable” masses are only marginally stable, and thus, only a relatively small fraction of these reach the detector.
In U.S. Pat. No. 8,389,929, a broader stability range is used to increase sensitivity. And, a deconvolution algorithm is used to quantify signals from various masses that may be stable at the same time. For example, temporal and spatial information at the detector can be used in the deconvolution process. Herein, such techniques are called broad-stability techniques or deconvolution techniques. However, the effectiveness of such techniques to increase sensitivity without sacrificing resolution can be reliant on maintaining careful control of the number of oscillatory field cycles experienced by the transmitted ions. Methods for controlling this parameter can be difficult to implement in practical instruments.
Therefore, it is desirable to provide new scanning techniques that address such problems when broad-stability techniques are used.